A variety of different implantable medical devices (IMD) are available for therapeutic stimulation of the heart and are well known in the art. For example, implantable cardiac defibrillators are used to treat patients suffering from ventricular fibrillation, a chaotic heart rhythm that can quickly result in death if not corrected. In operation, the defibrillator device continuously monitors the electrical activity of the heart of the patient, detects ventricular fibrillation, and in response to that detection, delivers appropriate shocks to restore a normal heart rhythm. Similarly, an automatic implantable defibrillator (AID) is available for therapeutic stimulation of the heart. In operation, an AID device detects ventricular fibrillation and delivers a non-synchronous high-voltage pulse to the heart through widely spaced electrodes located outside of the heart, thus mimicking transthoracic defibrillation. Yet another example of a prior art cardioverter includes the pacemaker/cardioverter/defibrillator (PCD) disclosed, for example, in U.S. Pat. No. 4,375,817 to Engle et al. This device detects the onset of tachyarrhythmia and includes means to monitor or detect progression of the tachyarrhythmia so that progressively greater energy levels may be applied to the heart to interrupt a ventricular tachycardia or fibrillation. Numerous other, similar implantable medical devices, for example a programmable pacemaker, are further available.
Regardless of the exact construction and use, each of the above-described implantable medical devices relies upon a power source or battery to provide requisite stimulation. Depending upon the particular application, the power source may be required to provided a stimulation energy of as little as 0.1 Joules for pacemakers to as much 40 Joules for implantable defibrillators. In addition to providing a sufficient stimulation energy, the power. source must possess low self-discharge to have a useful life of many months, must be highly reliable, and must be able to supply energy from a minimum packaged volume.
Suitable power sources or batteries for IMDs are virtually always electrochemical in nature, commonly referred to as an electrochemical cell. Acceptable electrochemical cells for implantable medical devices typically include a case surrounding an anode, a separator, a cathode and an electrolyte. The anode material is typically a lithium metal or, for re-chargeable cells, a lithium ion containing body. For most high-energy applications, the cathode is a solid material, such as silver vanadium oxide (SVO), and the electrolyte is a liquid, such as a lithium salt in combination with an organic solvent. Examples of acceptable lithium-based cells are disclosed in U.S. Pat. Nos. 5,458,997; 5,312,458; 5,298,349; 5,250,373; 5,221,453; 5,114,811; 5,114,811; 5,114,810; 4,964,877; 4,830,840; 4,391,729; 4,310,609; and 5,766,797. All of the foregoing patents are hereby incorporated by reference herein in their respective entireties. As is well known to those skilled in the art, a number of other lithium-based cells for IMDs are available, such as lithium/iodine, lithium/thionyl chloride, lithium/manganese dioxide, lithium/copper sulfide, lithium/carbon monofluoride, lithium/sliver chromate, etc.
Due to the importance of consistent, long-term performance, electrochemical cell manufacturers have expended great efforts in perfecting not only cell design, but also the manufacturing processes themselves. Nonetheless, as with any other manufactured product, electrochemical cell defects may arise from time-to-time. One failure mechanism associated with lithium/organic electrolyte batteries is an inter-electrode short caused by an oxidizable metal particle introduced at the cathode potential. With time, the metal particle corrodes and the metal ions are transported to the anode. The ions are then reduced at the anode and the metal plates out, often growing back through the separator to the cathode, causing an electrically conductive pathway (or "short"). As a result of this unexpected metallic corrosion-caused short, the cell may not perform properly.
In view of the importance of IMD cell reliability, manufacturers subject each and every cell produced to rigorous quality tests, both during manufacture as well as following final assembly. With respect to metallic contaminants, the generally accepted evaluation technique entails first performing a "bum-in" on the cell following assembly. Generally speaking, burn-in consists of pre-discharging the cell by a small percentage of its total capacity. In addition to the metallic contamination evaluation described below, burn-in serves to confirm overall cell integrity and to stabilize cell components. For example, a typical burn-in procedure involves discharge of the cell across a resistance on the order of 100 ohms or higher (i.e., a relatively high resistance for a period several hours). Following burn-in, the cell is then stored for many days. During the storage, the cell voltage is periodically measured. A change (e.g., decrease) in the measured voltage is indicative of metallic contamination or cell defect. Typically, an absolute, minimum voltage value is employed to identify defective cells. The actual storage time employed may vary from manufacturer to manufacturer. However, general industry standards require a storage time of 11-30 days. Obviously, this lengthy storage process greatly increases overall manufacture cycle time, thereby complicating the manufacturer's ability to meet expedited orders, project daily production values, identifying manufacturing quality issues, etc.